1. Field of the Invention
The field of art to which this invention pertains is the solid bed asorptive separation of isomeric toluidines. More specifically, the invention relates to a process for separating para-toluidine from the other toluidine isomers by employing a solid bed absorption system.
2. Background Information
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. As a few examples, a separation process disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491 uses a Type A zeolite to separate normal paraffins from branched chain paraffins, and processes described in U.S. Pat. Nos. 3,265,750 and 3,510,423 use type X or type Y zeolites to separate olefinic hydrocarbons from paraffinic hydrocarbons. In addition to their use in processes for separating hydrocarbon types, X and Y zeolites have been employed in processes to separate individual hydrocarbon isomers. As a few examples, adsorbents comprising X and Y zeolites are used in the process described in U.S. Pat. No. 3,114,782 to separate alkyl-trisubstituted benzene isomers; in the process described in U.S. Pat. No. 3,864,416 to separate alkyl-tetrasubstituted monocyclic aromatic isomers; and in the process described in U.S. Pat. No. 3,668,267 to separate specific alkyl-substituted naphthalenes. Because of the commercial importance of para-xylene, perhaps the more well-known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene from a mixture of C.sub.8 aromatics. In processes described in U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; 3,663,638; and 3,734,974, for example, adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers.
In contrast, this invention relates to the separation of non-hydrocarbons and more specifically to the separation of the isomers of toluidine. Substantial uses of toluidine are in dyes or dye intermediates, e.g., o-Toluidine is a precursor of azo dyes such as Acid Red 24, Solvent Yellow 3, Solvent Red 26 and Direct Red 62, while p-Toluidine is a precursor of Basic Red 9, Acid Green 25 and Acid Blue 78.
It is known from U.S. Pat. No. 3,069,470 to Fleck et al., to use type X zeolites for the separation of isomers of toluidine. The type X zeolite was found to be least selective for the para-isomer. Fleck et al. specifically states (column 2, lines 21-23), with regard to its process, that of three toluidine isomers, meta toluidine is preferentially adsorbed and paratoluidine is the least readily adsorbed. In Japanese Patent Application No. 56905/79, publicly disclosed on Nov. 20, 1980, it is disclosed that a solid adsorbent containing titanium oxide will selectively adsorb the p-isomer of toluidine.
We have discovered, contrary to the express teachings of the art, particularly Fleck et al., that certain cation exchanged X and Y zeolites are selective for para-toluidine over the other isomers, when used with the appropriate desorbent material.